With the increase in the use of portable electronic products such as video cameras, mobile phones, portable PCs, or the like, a secondary battery is commonly used as a main power source, and thus the importance of the secondary battery is growing.
Unlike a primary battery incapable of recharging, extensive research is undertaken regarding a secondary battery capable of charging and discharging so that they may be used in digital cameras, cellular phones, laptop computers, power tools, electric bicycles, electric vehicles, hybrid vehicles, large-capacity power storage apparatuses, or the like that are fast developing in the high-tech industry.
Particularly, since a lithium secondary battery has a higher energy density per unit weight and is capable of charging quickly compared to other secondary batteries such as lead accumulators, NiCd batteries, NiMH batteries, Li-Zinc batteries, or the like, the use of a lithium secondary battery is increasing.
A lithium secondary battery has an operating voltage of 3.6 V or more and used as a power source of portable electric apparatuses, or a plurality of lithium secondary batteries is connected in series or in parallel to be used in high-power electric vehicles, hybrid vehicles, power tools, electric bicycles, power storage apparatuses, UPS, etc.
Also, since a lithium secondary battery has an operating voltage three times higher than those of NiCd batteries or NiMH batteries and has excellent energy density characteristics per unit weight, the use of a lithium secondary battery is widely expanding.
Depending on the type of an electrolyte, a lithium secondary battery is categorized into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a polymer solid electrolyte. The lithium ion polymer battery is also divided into two types of batteries depending on the type of the polymer solid electrolyte: an all-solid lithium ion polymer battery containing no electrolyte solution and a lithium ion polymer battery containing an electrolyte solution and using a gel type polymer electrolyte.
Generally, a lithium ion battery using a liquid electrolyte is received in a cylindrical or prismatic metal can-shaped container and hermetically sealed for use. However, since a can-typed secondary battery using a metal can-shaped container is fixed in the shape thereof, electronic products having the can type secondary battery as a power source is limited in design, and has difficulty reducing its volume. Accordingly, a pouch type lithium secondary battery manufactured by receiving an electrode assembly and an electrolyte in a pouch packing made of a film, followed by sealing has been developed and in use.
However, a potential for explosion hazard may exist when a lithium secondary battery overheats, so ensuring the safety of a secondary battery is essential. The overheating of a lithium secondary battery is caused by various factors. One of the factors is the presence of an over-current in a lithium secondary battery. That is, if an over-current flows through a lithium secondary battery, heat is generated by Joule heating, and thus an internal temperature of the battery is quickly increased. Such an increase in temperature causes decomposition reaction of an electrolyte which brings about thermal running, causing the battery to inevitably explode. The over-current occurs when a sharp metal object penetrates a lithium secondary battery, or if an insulator between a cathode plate and an anode plate is destroyed by contraction of a separator being interposed between the cathode and anode plates, or if a rush current is applied to the battery due to an abnormal charge circuit or a load connected to the external.
In order to protect a lithium secondary battery from abnormalities such as an over-current, the battery is generally coupled to a protection circuit before use, and the protection circuit includes a fuse element which irreversibly disconnects a line where a charge or discharge current flows.
FIG. 1 is a circuit diagram showing the deposition structure and the operation mechanism of a fuse element in the configuration of a protection circuit coupled with a lithium secondary battery.
As shown in FIG. 1, the protection circuit includes a fuse element 10 for protecting a battery pack when an over-current occurs, a sense resistor 20 for sensing an over-current, a microcontroller 30 for monitoring the generation of an over-current and operating the fuse element 10 when an over-current occurs, and a switch 40 for switching the inflow of an operation current into the fuse element 10.
The fuse element 10 is installed in a main line connected to the outermost terminal of a cell assembly 20. The main line is a wire in which a charge current or discharge current flows. FIG. 1 shows that the fuse element 10 is installed in a high-voltage line (Pack+).
The fuse element 10 has three terminals, among these, two terminals are in contact with the main line in which a charge or discharge current flows, while the remaining one terminal is in contact with the switch 40. Also, the fuse element 10 includes a fuse 11 serially connected with the main line and melted at a predetermined temperature and a resistor 12 which applies heat to the fuse 11.
The microcontroller 30 monitors whether an over-current occurs or not by periodically detecting the voltage of both ends of the sense resistor 20, and when the occurrence of an over-current is determined, the microcontroller 30 turns on the switch 40. Then, the current which flows in the main line is bypassed to the fuse element 10 and applied to the resistor 12. Thereby, Joule heat generated from the resistor 12 is conducted to the fuse 11 to increase a temperature of the fuse 11, and when the temperature of the fuse 11 reaches the melting temperature, the fuse 11 melts, and thus the main line is irreversibly disconnected. When the main line is disconnected, an over-current no longer flows, thereby overcoming the problems associated with the over-current.
However, there are many problems in the conventional technology described above. That is, if there is a problem with the microcontroller 30, the switch 40 may not turn on even when an over-current occurs. In this case, since a current does not flow into the resistor 12 of the fuse element 10, there is a problem in that the fuse element 10 will not operate. In addition, a space for disposing the fuse element 10 is separately required in the protection circuit, and a program algorithm for controlling the operation of the fuse element 10 has to be loaded in the microcontroller 30. As a result, the space efficiency of the protection circuit deteriorates and the load of the microcontroller 30 increases.